1. Field of the Invention
The present invention relates to a method for ascertaining a temperature of a sheathed-element glow plug in an internal combustion engine, in which a temperature difference between the temperature of a glow plug heater inside the sheathed-element glow plug and the temperature at an arbitrary location on the sheathed-element glow plug is determined as a function of at least one operating parameter of the internal combustion engine and/or at least one operating parameter of the sheathed-element glow plug, and a device for carrying out the method.
2. Description of the Related Art
Sheathed-element glow plugs, which are used in internal combustion engines for igniting a fuel-air mixture, have a heater which preheats the cold sheathed-element glow plug to a temperature which is high enough for igniting the fuel-air mixture. However, the distribution of the temperature, starting from the heater and extending over the entire sheathed-element glow plug, is very inhomogeneous, resulting in temperature differences between the temperature of the heater inside the sheathed-element glow plug and the temperature at the surface of the sheathed-element glow plug.
Since the sheathed-element glow plug protrudes into the combustion chamber of the internal combustion engine, the surface of the sheathed-element glow plug is always cooled by the air stream which flows as a fuel-air mixture past the sheathed-element glow plug during dynamic operation of the internal combustion engine, so that the surface of the sheathed-element glow plug is never at the temperature of the heater inside the sheathed-element glow plug.
A method for operating a sheathed-element glow plug is known from published European patent document EP 1 719 909 B1, in which the sheathed-element glow plug is situated in the combustion chamber of the internal combustion engine. The cooling characteristics of the sheathed-element glow plug are computed as a function of the combustion temperature and the sheathed-element glow plug temperature. The temperature of the sheathed-element glow plug is computed from the electrical power consumed by the sheathed-element glow plug, while the combustion temperature in the combustion chamber is ascertained as a function of the air temperature, the cooling water temperature, the rotational speed of the internal combustion engine, and the load.
If the temperature of the sheathed-element glow plug is to be subject to control, this control is carried out as a function of the resistance of a current-conducting wire inside the sheathed-element glow plug, from which the actual controlled value of the temperature is ascertained. The resistance is higher the greater the temperature of the wire. Due to the temperature difference which occurs, the quality of the control of the sheathed-element glow plug temperature is inadequate, since this control is not based on a temperature which is actually present at the surface of the sheathed-element glow plug.